Ralf Rangnick is IMO a rubbish coach and have no idea how suited he is for Sports Director role.
Regarding Sports Director role - can be very helpful if manager and sports director are getting along. Manager is thinking more of short term and sports director more of a long term in this case and it can work.
Even in Premier League it can be very halpful. Look at Tottenham and QPR in recent years - Redknapp couldn't do as much whealing and dealing as he would like with Daniel Levy (more of a president for Spurs, but did some things which would work usual for Sports Director), and Hughes and Redknapp could do all they want in QPR.
If you have right manager (like Moyes) than you can clearly go without Sports Director. But if you get someone like Martin O'Neill... or Guardiola (who's transfers has been atrocious for Barcelona) then it can be really helpful.
Before someone corrects me - Guardiola was working with Sports Director in Barcelona, but after winning treble in first season he got so much power, that the other guy was sent to do basic paperwork. And then Eto'o for Ibra switch happened, plus Chigrinsky. That's how it ends when manager goes and does what he wants...